Type 2 immunity is characterized by production of interleukins (IL's) 4, 5, 13, 25, and IL-33, immunoglobulin E (IgE) eosinophilia, mastocytosis, basophilia, alternatively activated macrophages (AAM), smooth muscle hypercontractility, and mucus overproduction from activated goblet cells. CD4+ T helper 2 cells (TH2) that drive Type 2 immunity are critical for host-protection against parasitic helminths that cause significant morbidity and mortality in more than 2 billion people world-wide. TH2 cells are also central drivers of allergic diseases and asthma, which affects more than 100 million people world-wide. However the molecular mechanisms that initiate TH2 development in vivo remain unclear and are topics of much scientific debate. Here, we provide evidence that Trefoil factor 2 (TFF2), a mucus-stabilizing protein known to repair damaged epithelia, is central to the initiation of TH2 cells and development of Type 2 immunity. Hookworm infection rapidly induces TFF2 from lung epithelia, which acts to recruit interleukin (IL)-33-producing dendritic cells and macrophages into draining lymph nodes. TFF2 may recruit macrophages through CXCR4, the putative TFF2 receptor, but it is unclear whether CXCR4 is necessary for TFF2 function(s) in vivo. TFF2- treated-macrophages selectively expand TH2 cells from naove precursors through an IL-33-dependent, but MyD88-independent mechanism. It is unknown whether macrophages are necessary for TFF2-driven TH2 immunity. TFF2-deficient mice generate impaired primary and memory responses against the hookworms Nippostrongylus brasiliensis and Heligmosomoides polygyrus, respectively, which demonstrate the crucial role of this molecule in Type 2 immunity. However, it is unclear whether TFF2 functions solely through IL-33 dependent mechanisms. The goal of this project is to test our central hypothesis that: Trefoil factor 2 production from epithelia initiates TH2 immunity against hookworms through IL-33 induction specifically in macrophages. Specifically, this project will determine whether TFF2-mediated induction of Type 2 immunity: (1) is solely dependent upon macrophages or also requires dendritic cells and basophils, (2) requires IL-33 as a necessary and sufficient factor for host-protection and (3) requires CXCR4 signaling in epithelia or antigen- presenting cells. Successful completion of these aims will produce a better understanding of how Type 2 immunity is generated during parasite infection, which also bears relevance to therapies used to treat allergic disease. PUBLIC HEALTH RELEVANCE: Rapid induction of pathogen-specific immunity can determine life vs. death. We have uncovered a novel mechanism that initiates Type 2 immunity, which is responsible for protection against parasitic helminth infections and most allergic responses. Collectively, these diseases affect billions of people worldwide. This project will generate a better understanding of the molecular mechanisms that initiate Type 2 immunity.